The present invention relates to a current source for producing one or more output currents of magnitudes proportional to a variable input reference signal.
Some digital-to-analog convertors (DACs) include a current source for providing N output currents and a set of N switches, one output current and one switch corresponding to each bit of an input N-bit digital word to be converted to a voltage or current of proportional magnitude. Each switch connects one of the output currents to a load resistor when the corresponding bit is true so that the current supplied to the load resistor is equal to the sum of the output currents corresponding to true bits in the input word. When the magnitude of each output current is proportional to the weight of the corresponding bit in the word, the magnitude of the resulting current through the load resistor is proportional to the value of the N-bit word. For example, in standard binary encoding, assuming data bits are referenced from 1 to N in decreasing order of significance, the value represented by bit P (where P is any number from 1 to N) is twice the value represented by bit P+1. Therefore, the Pth output current is adjusted to equal I.0./(2.sup.P-1), where I.0. is a reference current magnitude. Thus the total current passing through the load resistor is equal to the value of the input digital word times I.0./(2.sup.N-1).
In "multiplying" DACs, the reference current I.0. magnitude is adjustable, proportional to an externally generated reference signal, such that the output voltage across the load resistor is nominally proportional to the product of the value of digital word and the magnitude of the reference signal. However, multiplying DACs of the prior art do not accurately maintain constant ratios of proportionality between the output currents and the reference signal as the reference signal is changed, particularly when operating with small power supply voltages. What would be useful is a current source which could produce one or more output currents, each of magnitude proportional to an applied reference signal wherein the constant of proportionality between the reference signal and each output current does not change when the reference signal magnitude is changed.